Cholesteric liquid crystals are widely used in conventional color display devices. It is simple to obtain chemical substances of changeable color over a wide temperature range by mixing various cholesteric liquid crystal compounds in suitable proportions. These cholesteric liquid crystal materials have been used for temperature measurement apparatus by coating them on surfaces or interposing them between a pair of transparent film sheets. They have been used for printing inks.
Conventional color display devices using cholesteric liquid crystal compounds are divided into two types; the reflection type and the transmission type. In reflection type devices, good contrast is not readily obtainable if the transmitted light is not absorbed by a black background. In transmission type devices, it is difficult to get a display device in which color change is clearly seen.